An alternative is not to stop uTorrent, but to block the traffic.
Assuming you're on Windows, and your torrent machine is always connected to your local network (i.e. it's not a notebook that's sometimes connected to public networks), the simplest method is to set up firewall rules to block uTorrent from having any network access unless the VPN is active.
- Configure the local network connection as a private network, and have all new networks encountered automatically considered public (that means any new VPN connection would be public). There was a simple way to do this with Windows 7, not sure about later versions.
- Create firewall rules to block uTorrent from sending any traffic via private networks, and allow it to send traffic via public networks.
- If you use the web UI then you will need to add an exception for the web UI port on the private network.
Once this is done if the VPN goes down uTorrent won't be able to send or receive any traffic at all (you'll see active torrents turn red).
Another option is to set up a second machine just for the VPN which forwards all traffic to the VPN Service
(this is a good use for an old netbook). Then configure the torrent machine to use the VPN machine as its gateway. This only works if all programs on the torrent machine need their traffic to go via the VPN (or the other programs can be configured to send via your router).